One of my first thoughts was to bend up a bunch of copper tubing and make buss bars for the interconnects for the batts that are grouped close together. But the impression I get from the EVDL is that solid buss bars are failure prone / loose prone, and that steered me off that path.
You might try using a damp rag around the cable, to help keep the cable from getting hot enough to wick the solder up. You should also be careful to not use any more heat than just enough to get the solder to flow at the joint of the wire and the pipe. Any more heat than that and it seems to create a oxidation on the copper which is resistant to the solder sticking.
We have a winner! I suggested exactly that to Ivan this evening.
PS - Ivan wasn't too happy with my home-made lugs (I will post some pics). He rightly pointed out that while the mechanical connection may be good, the flattened end of the copper pipe was much thinner than the lugs we have. So it wouldn't have had the same current capacity. Whether it would actually have been a problem or not in our application is another question...